1. Field of the Invention
The invention relates to an effective and economical strategy for lipid production. In particular, oleaginous yeast strains are used to hydrolyze biomass (e.g. wheat straw) that has been pretreated using dilute acid, in order to produce lipids. The lipids may be used as feedstock for producing biofuels.
2. Background of the Invention
Bio-fuel with its potential as a clean and renewable alternative to conventional fossil fuels has received increasing focus in recent years (Huang et al., 2009). However, successful bio-fuel commercialization economics depend on both large quantities and low cost of the feedstock. Lignocellulosic materials, due to their abundance and sustainability, have been attracting the attention of the bio-fuel production industry.
Wheat straw is an abundant by-product from wheat production. The average yield of wheat straw is 1.3-1.4 lb per lb of wheat grain (Montane et al., 1998). Based on the data from Food and Agricultural Organization, 55.8 million metric tons of wheat were produced in the USA in 2007 (world production, 606.0 million metric tons) (FAO, 2007). Wheat straw contains 35-45% cellulose, 20-30% hemicellulose and 8-15% lignin. And both cellulose and hemicellulose as the main components can be converted to fermentable pentoses and hexoses for bio-fuel production.
Due to the natural stability and resistance-to-bond-breaking properties of lignocellulosic materials, dilute acid pretreatment applied prior to hemicellulose hydrolysis greatly increases the effectiveness of the subsequent enzyme hydrolysis. The combination of acid pretreatment and enzymatic hydrolysis leads to the production of pentose and hexose species in the hydrolysate resulted from the dissolution of the hemicellulose.
The utilization of both pentose and hexose sugars present in dilute acid pretreated biomass hydrolysate is essential for economical bio-fuel production (Saha et al., 2005). Pentose, mainly xylose from hemicellulose, can be utilized for ethanol production by some either naturally occurring or genetically engineered strains such as, Thermoanaerobacter mathranii, Candida shehatae, Pachysolen tannophihts, Pichia stipitis, Saccharomyces Cerevisiae, Zymomonas mobilis and Escherichia coli (Agbogbo and Coward-Kelly, 2008; Ahring et al., 1998; Fu and Peiris, 2008). Hexose both from cellulose and hemicellulose is most widely used by S. cerevisia, which can give high ethanol yields and productivities in addition to a high ethanol tolerance (Olsson and Hahn-Haegerdal, 1996). However, S. cerevisia is unable to produce ethanol from xylose unless it is genetically engineered. Even in that case, low ethanol yield and the adverse effect of inhibitors contained in the hydrolysate often create technical challenges in using the process.
There is an ongoing need to develop new, improved and yet economical pocesses that use renewable lignocellulosic biomass to produce biofuels. In such processes, high efficiency utilization of pentose as well as hexose is required.